This invention relates to fuel cells and, in particular, to a sealing gasket for use between a high temperature fuel cell stack and manifold assembly. More specifically, this invention relates to a compliant gasket with an embedded resilient shim for maintaining a gas seal at the fuel cell stack end plate-bipolar plate interface.
A conventional fuel cell stack typically has several hundred fuel cells in series, forming several stack faces. In an externally manifolded stack, the fuel cells are left open on their ends and gas is delivered by way of manifolds or pans sealed to the respective faces of the fuel cell stack. Manifold assemblies are compressed against the fuel cell stack to prevent gas leaks between the manifolds and corresponding stack faces, as described in U.S. Pat. No. 6,413,665 owned by the assignee of the present application. The manifolds are also provided with a manifold gasket such as that disclosed, for example, in U.S. Pat. No. 4,467,018 to avoid gas leaks and to dielectrically isolate the fuel cell stack from the manifolds. The manifolds thus provide sealed passages for delivering fuel and oxidant gases to the fuel cells and directing the flow of such gases in the stack.
To maintain the performance of a fuel cell stack, the manifold gasket must continue to perform satisfactorily over the life of the stack. Thus, the manifold gasket must maintain its sealing effectiveness under the conditions required for operation of the fuel cell stack and for the duration of the life of the stack.
In high temperature fuel cell stacks, the aforesaid requirement that the manifold gasket retain its sealing effectiveness over the life of the stack is difficult to satisfy due to the environment of the stack which causes the dimensions of the stack and the stack components to change during long term operation. These changes are particularly evident in the bipolar plates of the stack fuel cells and the stack end plates which grow at different rates and, thus, exhibit an increasing disparity is size.
More particularly, during operation of a molten carbonate fuel cell stack, the bipolar plates of the stack grow over time due to carburization, whereby the bipolar plates absorb carbon from the carbon monoxide or carbon dioxide that is typically present in the stack fuel gas. For example, it has been found that in one type of molten carbonate fuel cell stack each bipolar plate will grow more than one tenth of an inch, or 0.21% of its total length, and more than one twentieth of an inch, or 0.19% of its total width, over an operating period of 14,500 hours. Bipolar plate growth in other types of molten carbonate fuel cell stacks is comparative depending upon the hours of operation.
The aforesaid growth of the bipolar plates in a molten carbonate fuel cell is not matched by corresponding growth of the stack end plates. This difference in growth distorts each end face of the stack. In particular, a step-like transition occurs at the interface between each end face of an end plate and the end face of the bipolar plates of the adjacent end cell. Also, in the center of the stack, the end face of the stack tends to bulge outward, due to the increased growth of the bipolar plates in the middle of the stack as compared to the bipolar plates at the ends of the stack.
FIG. 1 illustrates a worst-case example of the step-like transition at an end plate-bipolar plate interface of a stack. As can be appreciated, differential growth of the magnitude shown in FIG. 1 will cause the manifold gasket abutting the stack face to be placed under stress. Additional stress is caused at the center of the stack by the above-mentioned bulging. These stresses can lead to gasket failure.
Attempts have been made to reduce this stress by selecting materials for the bipolar plates which exhibit reduced growth. However, this has only partially alleviated the condition.
It is therefore an object of the present invention to overcome the above and other drawbacks of conventional manifold gaskets and, more particularly, to eliminate the stress on the gas seal at the stack-manifold interface during operation of the fuel cell stack.
It is another object of the invention to provide a modified fuel cell stack-manifold gasket that accommodates bipolar plate growth and maintains the gas seal between the stack face and manifold during operation of the stack.
It is another object of the present invention to provide a sealing gasket for use at the fuel cell stack-manifold interface that is reliable, inexpensive, and easy to manufacture and install.